Recently, medical personnel have found it useful to use robotic devices to assist in surgical procedures. A robotic device typically has a moveable arm that comprises one or more moveable links. A controller regulates activation of actuators that position the links. A surgical instrument attaches to a free end of the arm. The surgical instrument interfaces with a surgical site.
Conventionally, a force/torque transducer attaches between the free end of the arm and the surgical instrument. The conventional force/torque transducer cannot measure force or torque directly. Instead, strain on a micro-motion scale is measured and force is deduced based on the measured strain. From these strain measurements, the conventional force/torque transducer deduces forces and torques applied to the instrument. Specifically, as shown in FIG. 1, the force/torque transducer deduces three components of force (Fx, Fy, Fz) and three components of torque (Tx, Ty, Tz). The three components of force (Fx, Fy, Fz) represent axial loads along respective X, Y and Z-axes. The three components of torque (Tx, Ty, Tz) represent rotational loads about the respective X, Y and Z-axes.
The deduced forces and torques may result from various loads. For example, the load may be caused by the instrument pressing against tissue. Alternatively, the medical personnel setting the position and/or orientation of the instrument may apply the load. The conventional force/torque transducer deduces the resulting forces and torques and outputs signals to the controller. The controller processes the signals to determine control signals for determining a target position for the arm. Based on the determination of arm target position, the controller selectively activates the actuators in order to advance the arm to the target position.
A top view of the conventional force/torque transducer is illustrated at 10 in FIG. 2. The conventional force/torque transducer 10 has a fixed member 12 and a moveable member 14. The fixed member 12 is typically mounted to the free end of the arm. The moveable member 14 is secured to the surgical instrument. The moveable member receives the load applied to the surgical instrument. A plurality of spokes 16 connect the fixed and moveable members 12, 14. The spokes 16 bend in response to application of the load to the moveable member 14. The conventional force/torque transducer 10 in FIG. 2 has four spokes 16, however, such conventional transducers may include three spokes 16, and the like.
A plurality of strain gauges 18 attach to each spoke 16 for measuring the strain on the spoke 16. Often, as shown in FIG. 3, the strain gauges 18 attach to the top, bottom, and sides of each spoke 16 for measuring strain on the spokes 16 resulting from X, Y, and/or Z-axis loads.
Each spoke 18 and the strain gauges 18 associated with each spoke 19 collectively form a single-axis load cell in the transducer 10. As such, the conventional, typical force/torque transducer 10 in FIG. 3 comprises four separate load cells.
FIG. 4 illustrates in cross-section the response of the spokes 16 to a load applied axially along the Z-axis. Specifically, as shown in FIG. 4, the load is applied along the Z-axis to the moveable member 14 such that the moveable member 14 moves in a positive Z-axis direction. In response to this load, each of the spokes 16 behaves similarly. That is, each spoke 16 bends in the same direction. Similarly, in response to a load applied in the negative Z-axis direction, each spoke 16 bends in the same direction.
FIG. 5 illustrates from a top view the response of the spokes 16 to a load applied rotationally about the Z-axis. Specifically, as shown in FIG. 5, the load is applied about the Z-axis to the moveable member 14 such that the moveable member 14 rotates counter-clockwise. In response to this load, each of the spokes 16 behaves similarly. That is, each spoke 16 bends in the same direction. Similarly, in response to a load applied in the clockwise direction, each spoke 16 bends in the same direction.
The conventional force/torque transducer 10 is susceptible to providing unreliable measurements because of drift. Generally, drift is an abnormality in the measurements provided by a force/torque transducer.
In the conventional force/torque transducer 10, drift may occur during initialization of the conventional force/torque transducer 10. A strain gauge 18 is a resistor having a value that changes due to strain. As a resistor, the strain gauge 18 consumes power. The consumed power is transformed into heat. The heat of the strain gauge 18 is conducted locally to the material of the spoke 16 upon which the strain gauge 18 rests. An internal stress develops on the spokes 16 because the material of the spokes 16 expands, but is constrained. This stress results in a strain, which, in turn changes the electrical resistance of the strain gauges 18. Consequently, the change in resistance causes a false representation of applied force and/or torque and erroneous force/torque data.
FIG. 6 is a chart of X, Y and Z-axis forces deduced by the conventional force/torque transducer 10 during initial start-up. As observed, the Z-axis force exhibits significant drift, i.e., greater than three pounds, as compared to the X and Y-axis forces. Additionally, unlike the X and Y-axis forces that stabilize, the Z-axis force does not stabilize. Instead, the Z-axis force continuously varies in a range between 2.5 and 3.5 pounds.
FIG. 7 is a chart of X, Y and Z-axis torques deduced by the conventional force/torque transducer 10 during initial start-up. The X and Y-axis torques are stable and exhibit minimal drift. However, the Z-axis torque exhibits significant drift and does not stabilize. Specifically, the Z-axis torque drifts up to 1.4 inch-pounds. Thus, during initial start-up both the Z-axis force and the Z-axis torque suffer from significant drift.
The conventional force/torque transducer 10 is further susceptible to thermal drift. Thermal drift is often caused by thermo-expansion of the strain gauges 18. That is, current flowing through the strain gauges 18 causes the strain gauges 18 to heat up. The increase in the temperature causes the strain gauges 18 to expand locally. The local expansion causes a thermo-strain, which, in turn changes the electrical resistance of the strain gauges 18. Consequently, the change in resistance causes a false representation of applied force and/or torque and erroneous force/torque data.
In one experiment, the conventional force/torque transducer 10 was exposed to heat and the effects of thermal drift on the conventional force/torque transducer 10 were measured. FIG. 8 is a chart of X, Y and Z-axis forces deduced by the conventional force/torque transducer 10 during exposure to heat. The X-axis and Y-axis forces exhibited drift of less than 1 lb. However, the Z-axis force exhibited significant drift of nearly 12 pounds.
As apparent from these results, the conventional force/torque transducer 10 is highly susceptible to drift with respect to Z-axis forces and torques. That is, the conventional force/torque transducer 10 is prone to producing unreliable measurements with respect to Z-axis forces and torques.
The conventional force/torque transducer 10 is most susceptible to drift for Z-axis forces or torques because the spokes 16 exhibit similar bending forces in response to a rotational or axial load applied about/along the Z-axis, as described. Mainly, the negative effects of drift are summed because the bending forces are the same algebraic sign. For example, suppose the total Z-axis force (F) on the conventional force/torque transducer 10 as shown in FIG. 4 is calculated by summing the bending force (F1) on the left-side spoke 16 and the bending force (F2) on the right-side spoke 16. In response to the positive axial Z-axis load, both spokes exhibit positive bending forces. If the bending forces (F1) and (F2) are the same algebraic sign, e.g., positive, then the total Z-axis force is represented by F=|F1+F2|. The total force is the summation of the bending forces, rather than the difference between the bending forces. The same holds true when both bending forces are negative. As such, the conventional force/torque transducer 10 cannot offset bending forces for Z-axis forces or torques. Moreover, the conventional force/torque transducer 10 cannot negate the effects of drift on Z-axis forces or torques. As a result, summation of these similar bending forces causes a substantial amount of systemic mode noise when deducing the Z-axis forces or torques.
Numerous force/torque transducers, besides the conventional force/torque transducer 10 illustrated in FIGS. 2-5, exhibit bending forces of the same algebraic sign in response to applied forces or torques along/about the Z-axis. Such conventional force/torque transducers utilize various types of deforming members other than the spokes 16. For example, a conventional Stewart Platform force/torque transducer, such as the transducer illustrated in JP 2007-315878, equally suffers from the problems described above for the conventional force/torque transducer 10. Mainly, the force/torque transducer in JP 2007-315878 is also unable to adequately eliminate the aforementioned Z-axis drift issues. Specifically, the force/torque transducer in JP 2007-315878 is not capable of self-cancelling the bending forces for applied Z-axis forces. That is, all the deforming members go into tension when a Z-axis force is applied.
Therefore, there remains an opportunity to provide a force/torque transducer that at least solves the aforementioned problems.